Recently we saw 1/4 mile passes by both of the McLaren vehicles mentioned in the title. The 675LT when running a Lamborghini Huracan managed 10.234 @ 137.79 in the 1/4 mile. A 650S Spyder managed a 10.222 @ 138.29 which is both quicker and faster.

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Different runs on different days in different conditions will produce different results. Getting both cars together at the same time and at the same place is the only way to get a true comparison. Going from a roll removes launch variance as well.

The 675LT being lighter and having more power should mean on paper it wins. In practice, that is exactly what happens. The 675LT wins the race but it does get a bit of a jump. The 650S Spyder hangs in pretty well though.

The 675LT is likely experienced a bit more drag from its aero upgrades. What this race truly shows is that if you intend to do roll on racing with your McLaren you might as well save the cash and buy a used 650S (or even better an MP4-12C) and you will have no trouble with the 675LT which is built for the roadcourse.